It is necessary that a recording apparatus such as a laser printer provides high quality of an image, namely, extremely high resolution. In order to satisfy this need, the following method has been suggested. When recording data of 300 dpi, for example, created in a data processor, such as a host computer, inputted to a recording apparatus of 300 dpi, the recording data are temporarily recorded at resolution of 600 dpi by compensating a size, position, etc. of dots.
Meanwhile, in recent years, a tendency to high resolution of a recording apparatus has been remarkable. For example, in order to form an image at high resolution of 600 dpi, a recording apparatus having resolution of 600 dpi is connected to a host computer for creating recording data with recording density of 600 dpi so as to constitute a system as an image forming apparatus. Furthermore, an arrangement of a system which responds to a resolution of not less than 600 dpi is adopted.
However, as to a recording apparatus, the higher its resolution becomes, the more expensive its price becomes. for this reason, a system may be arranged such that a laser printer with resolution of 300 dpi is connected to a host computer for creating recording data at recording density of 600 dpi, for example, so as to form an image.
With such a system arrangement, when recording data with recording density of 600 dpi, for example, are simply recorded in a recording apparatus with resolution of 300 dpi, a size of an image doubles in length and breadth.
Therefore, for example, Japanese Unexamined Patent Publication No. 4-287566/1992 (Tokukaihei 4-287566) discloses an apparatus having a function that an image signal is converted according to a difference in resolution of a recording apparatus. With this function, when recording data created at high recording density of 600 dpi, for example, are thinned out so as to have recording density of 300 dpi, the thinned out recording data are outputted to a recording apparatus with resolution of 300 dpi so as to be recorded.
Here, an arrangement and an operation of a laser printer will be explained as one example of a recording apparatus referring to FIGS. 2 and 3 which are explanatory drawings of the present invention. First, as shown in FIG. 3, a laser beam emitted from a semiconductor laser 21 which is turned ON/OFF according to recording data is reflected successively by a polygon mirror 24 which is rotated and a reflecting mirror 28 so as to be irradiated on a surface of a photoreceptor drum 4.
At this time, when the polygon mirror 24 is rotated, the laser beam is scanned parallel to a center of the photoreceptor drum 4 and an electrostatic latent image according to recording data for 1 line is formed along the scanning direction, namely, a horizontal scanning direction according to ON/OFF operation of the semiconductor laser 21. Meanwhile, the photoreceptor drum 4 is also rotated in synchronization with the rotation of the polygon mirror 24. As a result, the electrostatic latent images by scanning of every one line are successively formed in a direction in which the photoreceptor 4 rotates, namely, a vertical scanning direction.
In such a manner, as shown in FIG. 2, the electrostatic latent image, which is formed on the surface of the photoreceptor drum 4 according to the recording data, is developed by toner in the developing unit so as to be visualized. Then, the toner image is transferred on recording paper and is fixed by a fixing unit 9 so that the image according to the recording data is formed on the recording paper.
Resolution of such a laser printer is determined by specification of an apparatus, such as a line space in the vertical scanning direction, namely, a rotating speed of the polygon mirror 24, a rotating speed of the photoreceptor drum 4 synchronized with the rotating speed of the polygon mirror 24. In the case where resolution in the vertical scanning direction is 300 dpi, resolution in the horizontal scanning direction, namely, a clock frequency of an optical system for turning ON/OFF the semiconductor laser 21 has been set such that the resolution in the horizontal scanning direction is equal to the resolution in the vertical scanning direction.
However, for example, in the case where in order to record recording data of 600 dpi in a laser printer with resolution of 300 dpi which is same as the above arrangement, the recording data are thinned out based upon a function for converting an image signal in the apparatus disclosed in the above-mentioned Publication, for example, an amount of data in the vertical scanning direction as well as an amount of data in the horizontal scanning direction are thinned out to 1/2. For this reason, an amount of data to be actually contributed to image formation in an amount of preliminarily created data responding to high resolution is greatly decreased, and as a result, an outline of an image, or a character, becomes jagged, thereby remarkably lowering printing quality.